The invention relates to a process for bending a component by means of mechanical force with the simultaneous selective heating of the component along the bending edge by means of a laser beam.
On bending in a V-die the metal sheet to be bent is firstly laid across a die of width w in a known manner, after which a bending punch is applied in the middle between the end points of the V acting as supports. The effect of the force produced by this stamp and the reaction forces produced thereby in the support points produce a bending moment in the metal sheet, by which the sheet bends, as a result of which the punch moves further in the direction of the vertex of the die. In the first phase of the bending process free bending is performed, which means that the bend line is determined only by the moment distribution in the metal sheet and neither the sides of the die nor the shape of the punch apart from the rounded edge in contact with the metal sheet have any influence. The influence of the die and the punch is only noticeable when the metal sheet due to the bending radius becomes detached from the original support points on the die and the latter move along the sides in the direction of the vertex. In this case gradually a positive contact is formed between the die and punch which produces the low springback characteristic of die bending or the precisely defined bend radius. This second phase of die bending is also called post-moulding or post-stamping. This phase is characterised by a significant increase in the force to be applied by the press.
The applicability of die bending is limited on the one hand by material properties such as fracture strain and yield stress and on the other hand by geometric conditions such as die width and the dimensions of the metal sheet. As the known theory of bending shows, the bending force required during free bending is proportional to the yield stress, the width of the metal sheet and the square of the thickness of the metal sheet so that with a maximum pressure force defined by the construction of the press there are limits with respect to the material properties and/or the die dimensions. In the stamping phase the force is only linearly dependent on the sheet thickness, whereby overall the force requirement is greater by a factor of 30 than during free bending.
A further limitation is the fracture strain, in that if the latter is exceeded tears appear on the bend edge. As the known theory of bending also shows, the critical load occurs in the outermost fibre of the bent component, whereby the strain produced there is dependent on the reciprocal value of the bending radius. A small bending radiusxe2x80x94and this is mostly desirable in practicexe2x80x94requires a high value for the fracture strain.
It is a known fact about materials that yield stress generally falls with component temperature, whereas fracture strain increases. Exceptions are materials, such as steel which have a distinct blue shortness, for which the former statement only applies above a limit temperature (in the order of 200xc2x0 C.) characteristic of the respective material.
The temperature behaviour of yield stress therefore makes it obvious to heat the material before or during the bending process in a known manner, so that the minimum possible bending radius or the required maximum force are reduced. The heating of the whole component is not practical however, as this requires a lot of energy, leads to distortion or oxidation and causes undesirable changes in the structure of the material. To avoid problems associated with heating the entire component, it is known to be possible to heat the component selectively by means of a laser beam, that is to supply energy only to the points at which the actual shaping process occurs in the component. In the case of die bending this is a linear region along the bending edge, whereby the outer zone, that is the region in which maximum tensile stress occurs in the metal sheet, reacts particularly favourably to heating, as tears are to be expected there first of all as a result of exceeding the tensile strength.
In U.S. Pat. No. 5,359,872 A and in DE 42 28 528 A1 according to these observations a metal sheet securely clamped on the one side and held on the other side by a robot is periodically traced over by a laser beam along the desired bend line at high speed, whereupon the robot applies a bending force and the bends the metal sheet along the preheated bend line. A disadvantage in this case however is that because of the pendulum movement of the laser beam the individual points on the bend line are only heated for a short time by the laser beam, producing a periodically fluctuating temperature field which in addition also shows phase displacement along the bend line. By means of a high frequency pendulum movement it can be ensured that these temperature fluctuations remain sufficiently small. In particular however due to the short acting high intensities localised overheating can occur, in extreme cases even to the stage of meltingxe2x80x94or it can result in too high cooling speeds, which affects the metallurgical properties of the metal sheet unfavourably in the region of the bend line. This problem becomes more serious the greater the laser power to be applied, i.e. the thicker the metal sheets or the longer the bend lines. A further disadvantage is that due to the geometric properties the laser beam only acts on the side with pressure on the component, whereby it is known that the critical zone on bending is the one with maximum tensile stress, which can cause the formation of tears on falling below the minimum bend radius. To produce the above described pendulum movement in addition to the optical converter device described in DE 42 28 528 A1, reflectors mounted helically on a support roller are also suitable, as shown in EP 0 536 683 A1.
In order to completely avoid disadvantageous periodical temperature fluctuations occurring according to U.S Pat. No. 5,359,872 A and DE 42 28 528 A1, the heating must be provided along the bend line by a laser beam shaped into a burn line. In order to shape the laser beam in this way numerous solutions are known. The method described in AT 138 837 B uses a combination of cylindrical mirrors and lenses in order to produce a linear focus with which a continuous metal sheet is heat-treated along its width, in order to influence its magnetic properties favourably. Other possibilities are described e.g. in EP 0 549 357 A1 which can be used both for laser beams and for radiation from conventional lamps. The high energy losses occurring in several of the optical systems introduced in EP 0 549 357 A1 for guiding the beam are thus clearly easy to accept, as the radiation necessary to process fabrics and plastics is low. Because of the geometric design of a device for bending and the high radiation output required the said processes are not suitable for shaping a linear heating zone.
According to the present invention the problem of periodically fluctuating temperature distribution is avoided, in that the laser beam is supplied as a linear radiation field to each point on the bend line. By this measure at all times on each point of the bend line conditions are identical, so that the bending process is completely homogeneous over the length of a component. To produce the linear radiation field and supply the radiation field to the bend line according to the invention the devices for shaping the radiation and bringing it into the devices required for bending for introducing the mechanical force are integrated. Preferably, the present invention can be applied for the process of die bending, so that mechanical forces are transferred by bending punch and bending die onto the component. An additional advantage of the present process is the fact that the laser beam now contacts the side of the component with the maximum tensile stress.
The feeding of the laser beam onto the surface of the component opposite the bending punch is performed in an embodiment of the invention by a system of cylindrical lenses and/or cylindrical mirrors. In this case the known laws of optics are used, which make it possible by means of two cylindrical lenses and/or mirror with axes in normal position to influence the divergence and the width diameter of a laser beam into two normal positioned transverse directions. By means of the corresponding design of the optical system according to the invention a linear radiation field and thereby a heating zone is produced along the bend line, which is calculated so that it is covered by the region in which the plastic deformation of the material to be bent is performed.
A further design of the invention is based on the use of a known slotted hollow conductor. A hollow conductor made of copper for example is able to transfer the beam of a CO2 laser with low losses, which is based on the low-loss multiple reflection inside the hollow conductor. By inserting a slot a level of power dependent on the relationship between the slot width and the circumference of the hollow conductor is decoupled from the hollow conductor, whereby this portion then contacts the material, for example steel, and is reflected. As reflection losses in steel are considerably greater than in copper, viewed over the entire length of the bending edge a large proportion of the power originally coupled into the hollow conductor is absorbed. To adjust to the power coupled per length unit into the component apart from the laser power, the gap width and the inner diameter of the hollow conductor are available, whereby both parameters can also vary with the length. To reduce losses in the hollow conductor or to ensure low losses over a long time period, there is the option of polishing the inside of the hollow conductor in a known manner and flushing it with a flow of gas in axial direction. As the axial power density in the hollow conductor is reduced as a result of absorption by the component, according to the invention it is also possible to couple the laser beam from both sides of the hollow conductor into the latter.
In order to produce a good contact between the component and the hollow conductor it is also possible according to the invention to press the hollow conductor by a support with elastic force against the bending edge, so that the latter always lies positively on the bend edge. In the region of the vertex of the die a recess is then provided which mounts the hollow conductor in the end phase of the bending process, so that in the end the bending die, bending punch, metal sheet and hollow conductor are in positive contact with one another.
In a further embodiment of the invention several linear radiation fields produced by a cylindrical optical system or by a hollow conductor are arranged consecutively or overlapping. The larger number of laser beams necessary for this purpose is produced either by a single laser beam of high power divided according to known methods or by several lasers of low power.
It is also possible according to the invention by selecting the mode of operation of the laser, such as continuous wave or pulsed, to adjust the heating and cooling flows in the material. All points of the bend line are in this case contact simultaneously with laser radiation.
Lastly, a process is advantageous in which linear radiation fields are guided through the upper and lower die of the bending die, and to form an even heating zone for the opposite surfaces of the component are fed along the bend line, whereby for the bending due to the properties of the material only certain suitable materials can be processed and also components which have a greater thickness.